1. Field of the Invention
The present invention relates to a copper interconnection structure which is used in a very large scale integrated circuit (VLSI) and a method of preparing the copper interconnection structure.
2. Description of the Prior Art
The line width of an electrode interconnection structure has recently become substantially narrower in response to the complexation of the structure of the VLSI. Hitherto, an aluminum or aluminum alloy has been used as an electrode interconnection material. However, for example, as the line width of an aluminum alloy pattern is narrowed to 0.3 .mu.m or less, its electromigration resistance or stressmigration resistance is lowered. With this, the quality of the interconnection structure tends to be seriously deteriorated. In view of this, instead of aluminum, a metal having a high melting point such as molybdenum or tungsten has been proposed to be used as an electrode interconnection material. However, electrical resistance of rnolybdenum or tungsten is very high as compared with that of aluminum. Therefore, molybdenum or tungsten is not suitable for a so-called high-speed device which is requisite for the latest VLSI. Thus, as is disclosed in a monthly journal of "Semiconductor World", June, 1988 issue, 89 p-93 p, there is another proposal to use copper as an electrode interconnection material. Copper is low in electrical resistance and has a high electromigration resistance. However, copper has the following drawback.
Copper is oxidized at a temperature of about 200.degree. C. in an atmosphere containing at least several percent of oxygen. In fact, in a wafer process such as an insulating film formation by CVD method or annealing, the patterned copper is exposed to a high temperature in an atmosphere containing oxygen. Unlike aluminum, an oxidized surface layer serving as a barrier for suppressing a further oxidation is not produced on copper. In fact, oxygen diffuses into the interior of the patterned copper so as to oxidize the same. With this, electrical resistance of the patterned copper drastically increases, thereby making copper difficult to be used as an electrode interconnection material of the VLSI.